Ricardo Martin Holdo

University of Georgia | UGA · Odum School of Ecology

Fires in grassy ecosystems consume vegetation and initiate high‐quality regrowth, which results in pyric herbivory when mammalian grazers concentrate feeding in recent burns. For environmentally transmitted parasites with transmission mechanisms linked to vegetation structure, fire should exert direct effects on parasites, as well as indirect effec...

Root‐based functional traits are relatively overlooked as drivers of savanna plant community dynamics, an important gap in water‐limited ecosystems. Recent work has shed light on patterns of trait coordination in roots, but less is known about the relationship between root functional traits, water acquisition, and plant demographic rates. Here, we...

Savannas cover one‐fifth of the Earth's surface, harbour substantial biodiversity, and provide a broad range of ecosystem services to hundreds of millions of people. The community composition of trees in tropical moist forests varies with climate, but whether the same processes structure communities in disturbance‐driven savannas remains relatively...

Large herbivores play unique ecological roles and are disproportionately imperiled by human activity. As many wild populations dwindle towards extinction, and as interest grows in restoring lost biodiversity, research on large herbivores and their ecological impacts has intensified. Yet, results are often conflicting or contingent on local conditio...

Models of tree–grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade‐off between wa...

Savannas cover a significant fraction of the Earth's land surface. In these ecosystems, C3 trees and C4 grasses coexist persistently, but the mechanisms explaining coexistence remain subject to debate. Different quantitative models have been proposed to explain coexistence, but these models make widely contrasting assumptions about which mechanisms...

Purpose Soil moisture availability is a key driver of the relative abundance of trees and grasses in savannas. Fine root morphology influences water acquisition and transport, yet differences in fine root traits between trees and grasses have not been comprehensively quantified across an entire suite of coexisting tree and grass species. Methods We...

In savanna ecosystems, fire and herbivory alter the competitive relationship between trees and grasses. Mechanistically, grazing herbivores favor trees by removing grass, which reduces tree‐grass competition and limits fire. Conversely, browsing herbivores consume trees and limit their recovery from fire. Herbivore feeding decisions are in turn sha...

Savanna tree cover often exhibits sudden discontinuities across space. It has been proposed that local spatial processes imposed by variation in tree cover itself (as opposed to by external drivers such as edaphic variation) can reinforce such discontinuities. Despite this, we generally lack data on tree demography and the environmental drivers aff...

Context Studies of tree cover and spatial pattern in tropical savannas often focus on coarse-scale effects of climate, disturbance, and herbivory. Local hydrological, topographic, or edaphic factors, however, play an important role in determining resource availability for trees, but we often lack data for these variables at fine spatial resolution....

Abiotic stresses, physiological dysfunction, forest stand dynamics and primary tree attackers (native and non‐native) are all recognized as important contributors to both anomalous tree mortality and background tree mortality, and thus as important influences on biogeochemical cycling and habitat for associated terrestrial organisms. Opportunistic...

Positive biodiversity–ecosystem function relationships (BEFRs) have been widely documented, but it is unclear if BEFRs should be expected in disturbance‐driven systems. Disturbance may limit competition and niche differentiation, which are frequently posited to underlie BEFRs. We provide the first exploration of the relationship between tree specie...

Vegetation states in savannas are highly sensitive to tree growth rates, which determine whether individual trees can “escape” periodic disturbances. Resprouting trees have lopsided shoot:root ratios and are often multi‐stemmed, and these variables can modify post‐disturbance growth rates and therefore the probability of escape. To date, few studie...

The idea that tropical forest and savanna are alternative states is crucial to how we manage these biomes and predict their future under global change. Large-scale empirical evidence for alternative stable states is limited, however, and comes mostly from the multimodal distribution of structural aspects of vegetation. These approaches have been cr...

Significance We develop a biogeographic approach to analyzing the presence of alternative stable states in tropical biomes. Whilst forest–savanna bistability has been widely hypothesized and modeled, empirical evidence has remained scarce and controversial, and here, applying our method to Africa, we provide large-scale evidence that there are alte...

PREMISE: Belowground functional traits play a significant role in determining plant water use strategies and plant performance, but we lack data on root traits across communities, particularly in the tropical savanna biome, where vegetation dynamics are hypothesized to be strongly driven by tree–grass functional differences in water use. METHODS: W...

In savannas, partitioning of below‐ground resources by depth could facilitate tree–grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root‐niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the de...

A proper mechanistic understanding of how whole-tree transpiration varies as a function of environmental conditions is essential for predicting how vegetation will respond to climate change, particularly to changes in soil moisture dynamics expected under novel rainfall regimes. Whole-plant transpiration studies in trees rely on Granier-type sap-fl...

Although there is a well‐known association between tree cover and soil texture in savannahs, the hydrological drivers of tree cover variation have not been systematically explored, particularly in parallel with factors such as fire, herbivory, and tree–grass interactions. The relationship between hydrological factors and tree cover is important for...

Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti-Mara ecosystem, document multide...

Aim In tropical Africa, savannas cover huge areas, have high plant species richness and are considered as a major natural resource for most countries. There is, however, little information available on their floristics and biogeography at the continental scale, despite the importance of such information for our understanding of the drivers of speci...

Savanna ecosystems span a diverse range of climates, edaphic conditions, and disturbance regimes, the complexity of which has stimulated long‐standing interest in the mechanisms that maintain tree–grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the t...

Rooting differentiation between established trees and grasses has been well documented in savannas, but it remains unclear to what extent tree-grass rooting differences affect competition between newly established seedlings and grasses. To examine this question, a greenhouse experiment was conducted at the University of Missouri, USA. Twenty 3-mo-o...

A significant fraction of the terrestrial biosphere comprises biomes containing tree–grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree–grass vertical root separation occur...

Bastin et al. (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, lead...

Savanna tree species vary in the magnitude of their response to grass competition, but the functional traits that explain this variation remain largely unknown. To address this gap, we grew seedlings of 10 savanna tree species with and without grasses in a controlled greenhouse experiment. We found strong interspecific differences in tree competiti...

Tree recruitment in savannas proceeds in multiple stages characterized by successive filters occurring at the seed and seedling stages. The “demographic bottleneck” hypothesis suggests that such filters ultimately restrict tree density and prevent trees from dominating grasses in savannas, but many of the demographic transitions underlying this ass...

Savannas are spatially diverse, variable and are susceptible to high rates of disturbance from fire and herbivory. There is significant interest in woody cover dynamics in relation to disturbance regimes. Less effort has been devoted to understand processes that drive tree community composition. In this study, tree species composition data collecte...

Much concern has been expressed about the sustainability of fuelwood harvesting in Africa. Most models predict that demand will outstrip supply within a few decades, resulting in severe deforestation. However, despite substantial impacts of harvesting on woody vegetation structure, the ‘fuelwood crisis’ predicted since the 1970s has not materialize...

Generalizations about the drivers of tree demography in tropical savannas continue to prove difficult because of the complex and dynamic interactions involved, and because multi‐year data sets spanning meaningful gradients in potential drivers are lacking. Overstorey trees play disproportionate roles in the long‐term dynamics and functioning of sav...

Climate change decoupling mutualism Many coevolved species have precisely matched traits. For example, long-tongued bumblebees are well adapted for obtaining nectar from flowers with long petal tubes. Working at high altitude in Colorado, Miller-Struttmann et al. found that long-tongued bumblebees have decreased in number significantly over the pas...

This research focused on understanding the main spatial driver(s) of species distribution as a function of multiple environmental gradients in the Serengeti savannas.

Vultures provide critical ecosystem services, yet populations of many species have collapsed worldwide. We present the first estimates of a 30-year Pan- African vulture decline, confirming that declines have occurred on a scale broadly comparable with those seen in Asia, where the ecological, economic, and human costs are already documented. Popula...

Wildebeest are abundant in Serengeti. Comparisons of specific attributes of wildebeest with other sympatric herbivores are necessary. Each section builds on the results of the previous sections to form a complete story that combines biology, behavior, and the geomorphology of the Serengeti ecosystem; it explains how a single species can outnumber a...

Tree-grass competition is recognized as an important ecological process in savannas and woodlands, yet we only have a limited understanding of how it varies across tree species, particularly in relation to water use and root morphology. We grew seedlings of three African savanna tree species either with or without grasses in a greenhouse experiment...

Life-history trade-offs and the costs of reproduction are central concepts in evolution and ecology. Episodic climatic events such as drought and extreme temperatures provide strong selective pressures that can change the balance of these costs and trade-offs. We used size-structured matrix models parameterized from field and laboratory studies to...

It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functio...

Vultures provide critical ecosystem services, yet populations of many species have collapsed worldwide. We present the first estimates of a 30-year Pan-African vulture decline, confirming that declines have occurred on a scale broadly comparable with those seen in Asia, where the ecological, economic, and human costs are already documented. Populat...

For many grassland and savanna ecosystems, water limitation is a key regulator of individual plant, community and ecosystem processes. Maximum rooting depth is commonly used to characterize the susceptibility of plant species to drought. This rests on the assumption that deep‐rooted plant species would have a greater total volume of soil water to e...

Tree size distributions are the outcome of demographic processes and disturbance events, and size distribution analysis provides a useful tool for understanding pattern and process in tree population dynamics. Demographic bottleneck mechanisms such as fire “traps” are important for driving tree cover dynamics in savanna systems, and bottlenecks mig...

Questions What is the pattern of compositional similarity for woody plants across the demographic bottleneck (i.e. canopy trees >2 m vs understorey trees <2 m) commonly observed in savannas? Does compositional similarity between woody plants in the canopy and understorey change across environmental gradients, and if so, which resource or disturbanc...

Background/Question/Methods In plant communities characterized by high productivity and high species turnover, such as many C4 savannahs, identifying the relative influence of biotic and abiotic controls is challenging because of the number and interactivity of different controlling agents. African savanna elephants (Loxodonta africana africana)...

Areas of locally intense and frequent grazing, or ‘hotspots’, are pervasive features in tropical grasslands and savannas. In some ecosystems, hotspot presence is clearly associated with edaphic factors (e.g., high clay content and elevated soil fertility), such as those that develop in abandoned cattle bomas. Studies in a range of other savanna eco...

Savannas are highly heterogeneous tree‐grass mixtures, and the structural variation imposed by a discontinuous canopy cover results in spatial variation in soil properties such as plant‐available nutrients, temperature and soil moisture. Many savannas are also dominated by large vertebrate herbivores, which impose a different suite of effects on so...

Mixed oak stands in the Ozark Highlands of southern Missouri were revisited eight years after a severe episode of red oak decline to determine which predictor variables, collected in 2003, best predicted subsequent tree growth and mortality patterns. Between 2002 and 2009, the mortality rate was 5% (0.625% annual mortality rate), generally below pr...

The two-layer hypothesis of tree-grass coexistence posits that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers while trees have exclusive access to deep water. The lack of clear differences in maximum rooting depth between these two functional groups, however, has caused this model to fall out of f...

Source code. (ZIP)

Coefficients of stochastic rainfall generator as a function of mean annual precipitation (MAP) across four North American LTER sites. (a) rate λ of the exponential distribution describing interarrival tS1imes between precipitation events, (b) mean μ and (c) standard deviation σ of the lognormal distribution describing event size. Key to LTER sites:...

Background/Question/Methods The Walter model of tree-grass coexistence posits that trees and grasses are able to coexist in stable equilibrium as a result of rooting differences. Grasses are assumed to be competitively dominant in shallow soil layers while trees have access to deep moisture. A key premise of this two-layer model is that exclusive...

Reliable mapping of tree cover and tree-cover change at regional, continental, and global scales is critical for understanding key aspects of ecosystem structure and function. In savannas, which are characterized by a variable mixture of trees and grasses, mapping tree cover can be especially challenging due to the highly heterogeneous nature of th...

Fire exerts clear direct effects on savannah vegetation dynamics, but the indirect effects of fire, for example via changes in soil fertility, are less clear. In fire-dominated nutrient-poor savannahs, there is little evidence that fire affects plant-available nutrients, but plant performance on soils with contrasting fire histories has not been in...

Theoretical models of tree-grass coexistence in savannas have focused primarily on the role of resource availability and fire. It is clear that herbivores heavily impact vegetation structure in many savannas, but their role in driving tree-grass coexistence and the stability of the savanna state has received less attention. Theoretical models of tr...

Background / Purpose: Grazing lawns are highly-utilized patches that are key resource areas for grazing animals. Lawns are thought to be maintained through complex interactions between plants, soils, fire, fungi and grazers. We examined how below-ground factors, plant community turnover and intraspecific plant species variation function within la...

Animal population‐level phenomena are often inferred from large tracking data sets obtained from only a few individuals. Two key challenges are to understand how these two scales are related, and to identify the factors that influence the extent to which small samples consisting of a few individuals can predict spatial patterns at the population sc...

Background/Question/Methods Grazing lawns are highly‐utilized grassland patches that represent key resource areas for large herbivores. Within these lawns exists the potential for complex interactions among herbivores, plants, fungi, and soils. It is essential that we understand the mechanisms that presently maintain lawns today so that we can bet...

Background/Question/Methods Two models are usually invoked to explain tree-grass coexistence in savanna ecosystems: the ‘two-layer’ hypothesis and the ‘demographic bottleneck hypothesis.’ The two-layer model argues that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers and trees using deeper water....

Questions What are the independent and interactive effects of fire and tree canopies on soil nutrient and C pools in savannas? Does fire differentially affect total and labile pools of C and N ? How do these effects differ between nutrient‐poor, broad‐leaved savannas on sandy soils of granitic origin, and nutrient‐rich, fine‐leaved savannas on clay...

Background/Question/Methods Savanna is widespread globally and is the dominant African biome. Tree cover is a key attribute of ecosystem structure, a dominant driver of ecosystem function and biodiversity, and plays an important role in socio-ecological systems. Understanding process and pattern in savanna tree cover dynamics is critical to the he...

Background/Question/Methods A recurring question in community ecology is: why do savannas have so few trees? In other words, what keeps trees from colonizing and growing in the open patches of grassland, which are theoretically capable of becoming dense woodland? Theories of tree limitation in savannas have focused either on bottom-up constraints,...

Background/Question/Methods In the Southeastern United States, two families of “giant salamanders” have evolved to live in a variety of aquatic habitats. Amphiumidae and Sirenidae each contain species that diverged in body size and are capable of surviving periodic droughts by aestivating in wetland sediments. Although both families have followed d...

The Serengeti wildebeest migration is a rare and spectacular example of a once-common biological phenomenon. A proposed road project threatens to bisect the Serengeti ecosystem and its integrity. The precautionary principle dictates that we consider the possible consequences of a road completely disrupting the migration. We used an existing spatial...

This chapter discusses how models, combined with modern data sources and statistical methods, can be used to test different hypotheses about the causes of migration. Mathematical formulations for migration are presented and the ecological mechanisms are discussed that could spontaneously have given rise to migration-like patterns of space use from...

Tanzania's iconic national park must not be divided by a highway, say Andrew Dobson, Markus Borner, Tony Sinclair and 24 others. A route farther south would bring greater benefits to development and the environment.

Migratory ungulates may be particularly vulnerable to the challenges imposed by growing human populations and climate change. These species depend on vast areas to sustain their migratory behavior, and in many cases come into frequent contact with human populations outside protected areas. They may also act as spatial coupling agents allowing feedb...

R and WinBUGS computer code used for the BSS model. (0.06 MB DOC)

Time-series data for model variables used in the analysis. (0.04 MB DOC)

Tree cover is a fundamental structural characteristic and driver of ecosystem processes in terrestrial ecosystems, and trees are a major global carbon (C) sink. Fire and herbivores have been hypothesized to play dominant roles in regulating trees in African savannas, but the evidence for this is conflicting. Moving up a trophic scale, the factors t...

Multiple hypotheses have been proposed to explain the annual migration of the Serengeti wildebeest, but few studies have compared distribution patterns with environmental drivers. We used a rainfall-driven model of grass dynamics and wildebeest movement to generate simulated monthly wildebeest distributions, with wildebeest movement decisions depen...

Vertebrate herbivores and fire are known to be important drivers of vegetation dynamics in African savannas. It is of particular importance to understand how changes in herbivore population density, especially of elephants, and fire frequency will affect the amount of tree cover in savanna ecosystems, given the critical importance of tree cover for...

The pattern of coarse-root distribution was analysed in the woody plant community along a 200-m edaphic gradient on a Kalahari sand woodland catena in Zimbabwe. The root systems of 45 trees and shrubs were excavated, mapped, and digitized to analyse rooting depth and architecture. Patterns of change in the above-ground community were also identifie...

African elephants Loxodonta africana (Blumenbach) may profoundly affect vegetation and associated animal bio-diversity in savannas (Conybeare 2004, Skarpe et al. 2004). Understanding the patterns of habitat use by elephants is crucial to predict their impacts on ecosystems (Ben-Shahar 1993, Nelleman et al. 2002), particularly now that many populati...

Fire, elephants, and frost are important disturbance factors in many African savannas, but the relative magnitude of their effects on vegetation and their interactions have not been quantified. Understanding how disturbance shapes savanna structure and composition is critical for predicting changes in tree cover and for formulating management and c...

Herbivores can play a key role in affecting ecosystem function, but their direct and indirect effects are often confounded with each other and have rarely been dissected. Predictions for open systems, i.e. those with cross‐habitat nutrient fluxes and dispersal, may differ from those expected in closed systems, where no such transfers occur, but the...